The present invention relates to a process for the coating of the flow channels of a honeycomb form catalyst carrier with a dispersion coating.
Catalytic converter carriers in honeycomb form are in large measure used for the creation of automobile exhaust catalytic converters. They have a cylindrical shape, and are filled with a multitude of flow channels parallel to the axis of the cylinder for the exhaust of internal combustion engines. The cross-sectional shape of the catalyst carrier depends on the requirements of the vehicle design. In wide use are catalytic converters with round, elliptical or triangular shaped cross-sections. The flow channels mostly have a square cross-section and are arranged in a tight grid over the entire cross-section of the catalytic converter. According to the particular application scenario, the cell density of the flow channels varies between 10 and 120 cm.sup.-2. Honeycomb carriers with cell densities up to 250 cm.sup.-2 and more are being developed.
Predominantly used for the purification of auto exhaust gas are catalyst carriers that are obtained through extrusion of ceramic masses. Alternatively, catalyst carriers made out of corrugated and wound up metal foils are available. Ceramic catalyst carriers with cell densities of 62 cm.sup.-2 are still predominantly used today for the exhaust purification of passenger vehicles. The cross-sectional dimensions of the flow channels in this case amount to 1.27.times.1.27 mm.sup.-2. The wall thicknesses of these catalyst carriers lie between 0.1 and 0.2 mm.
For the most part, finely distributed platinum group metals, which can be changed in their catalytic effect through compounds of base metals, are used for the conversion of pollutants contained in automobile exhaust--such as carbon monoxide, hydrocarbons, and nitrogen oxide--into innocuous compounds. These catalytically active components must be deposited on the catalyst carriers. However, it is not possible to guarantee the required fine distribution of the catalytically active components through deposition of these components on the geometric surfaces of the catalyst carriers. This applies in the same way to non-porous metallic catalyst carriers as well as porous ceramic ones. A sufficiently large surface area for the catalytically active components can only be made available through application of a support coating of fine particulate, high surface area materials.
Thus the present invention relates to a process for the application of such a support coating on the inner surface of the flow channels of honeycomb form catalyst carriers. Within the framework of this invention, the support coating for the catalytically active components is designated as a dispersion coating. The dispersion coating consists of fine particulate high surface area materials and is produced under application of a so-called coating dispersion. The coating dispersion for the most part involves a slurry of the fine particulate materials in water.
Various processes for the deposition of the dispersion coating on the catalyst carrier under application of the coating dispersion are known from the state of the art. After the coating process, the catalyst carriers are dried and then calcined for the strengthening of the dispersion coating. Then the catalytically active components are introduced into the dispersion coating through impregnation with mostly aqueous solutions of precursor compounds of the catalytically active components. Alternatively, the catalytically active components can be added to the coating dispersion itself. In this case, an additional impregnation of the prepared dispersion coating with the catalytically active components is no longer necessary.
For effective purification of the exhaust gases of internal combustion engines, the volume of the catalyst carriers must have a sufficient dimension. Usually a ratio of the engine cylinder displacement to the volume of the catalyst carrier of 1:2 to 2:1 is chosen. Thus catalyst carrier typical for automobiles have a volume of about 1 liter with a diameter of 100 mm (4 inches) and a length of 152 mm (6 inches). The dry weight of the dispersion coating on such catalyst carriers is from 50 to 200 g/l volume of the catalyst carrier. With a cell density of 62 cm.sup.-2, this corresponds to a calculated coating thickness of the dispersion coating of 20 to 80 .mu.m. Because of the capillary forces, the dispersion coating as a rule is nevertheless very unevenly distributed over the cross section of the flow channels, with strong accumulations of coating material in the corners of the channels and relatively thin coating thicknesses on the centers of the channel walls.
Processes for the application of the dispersion coating on the catalyst carriers must have a high productivity with low amounts of rejects. They must therefore make it possible to apply the entire quantity of coating on the catalyst carrier in a single operational cycle. Multiple coatings to attain the required coating thickness should be avoided. Moreover, the coating processes must assure that the coating material does not clog the flow channels. Furthermore it is generally required from such coating processes that they avoid coating the outer covering of the catalyst carrier. In so doing, expensive coating material and potentially expensive catalytically active components can be saved.
GB 1 515 733 describes a coating process for ceramic catalyst carriers. The porous catalyst carriers are inserted upright, that is, with vertically oriented flow channels, in a pressure tight coating tank and degassed through application of a partial vacuum of 0.84 bar (25 inch mercury column). Next the coating dispersion is poured over the upper face of the catalyst carrier into the coating tank and pressed into the pores of the catalyst carrier through application of overpressure. After withdrawal of the overpressure and opening of a release valve in the base of the coating tank, the excess coating dispersion flows out of the flow channels of the catalyst carrier. Finally, any channels blocked by coating dispersion are blown open with compressed air blown from top to bottom. The cycle time of this coating process amounts to less than 11/2 to 2 minutes.
U.S. Pat. No. 4,208,454 likewise describes a process for the coating of porous ceramic catalyst carriers. The catalyst carriers to be coated are placed with their lower face on the opening of a collection tank in which the pressure is reduced by means of a high volume blower by 5 to 16 inches of water column in relation to the atmospheric pressure. This partial vacuum is held constant during the entire coating time. A predetermined volume of the coating dispersion is distributed over the upper face of the catalyst carrier and steadily run through the flow channels into the collection tank. The suction cycle is maintained for about 30 seconds. After the first 5 seconds, the entire quantity of coating is run through the catalyst carrier. During the remaining time, the air flowing through the flow channels ensures that any flow channels blocked by coating dispersion are opened. The quantity of coating remaining on the catalyst carrier can be influenced by the duration of the overall suction time and by the level of the partial vacuum. The axial uniformity of the coating on the catalyst carrier can be improved by turning the catalyst carrier after about half of the suction time and then suctioning in the opposite direction. With this process, coating dispersions with 30 to 45% solids content as well as a viscosity of from 60 to 3000 cps can be processed. The preferred solids content lies at 37% by weight and the preferred viscosity at 400 cps. The reproducibility of the quantity of coating is given at +/-5% in this process.
EP 0157 651 likewise describes a process for the coating of ceramic catalyst carriers with a predetermined quantity of a coating dispersion. To accomplish this, the pre-weighed quantity of the coating dispersion is poured into an open wide tank and the catalyst carrier with its bottom face dipped in the dispersion. Then the dispersion is sucked into the flow channels through application of a light partial vacuum. In order to improve the axial uniformity of the coating, it is recommended here also to have the coating process proceed in two steps.
In the first step, only about 50 to 85% of the entire quantity of coating is poured into the tank and sucked into the catalyst carrier. Afterwards, the catalyst carrier is turned and the remaining quantity of coating is suctioned in the opposite direction. This coating process requires no separate step for the opening of any closed flow channels. The cycle time of this process amounts to less than 1 minute. With this process, coating dispersions that have a solids content of between 35 and 52% and a viscosity of between 15 and 300 cps can be processed.
U.S. Pat. No. 5,182,140 describes a process for the coating of ceramic and metallic catalyst carriers. In this process, the coating dispersion is pumped from underneath into the vertically placed catalyst carrier until the dispersion reacher a level completely above the upper face of the catalyst carrier. Then the coating dispersion is removed from the substrate through application of compressed air on the upper face of the catalyst carrier. In this way, any flow channels that are still closed are blown open. According to Example 1 of this patent document, a level of the coating dispersion of 2 cm over the upper face of the catalyst carrier is used. The compressed air for the expulsion of the coating dispersion from the flow channels is introduced in two successive stages of pressure. During the first 2 seconds after filling the catalyst carrier, the coating dispersion is acted upon with compressed air of 3.7 bar. This high pressure is sufficient to expel the coating dispersion completely from the flow channels during the available 2 seconds. Afterwards, the compressed air pressure is reduced to 0.37 bar and the catalyst carrier acted upon twice with this pressure for 0.5 seconds each time. With this process, coating dispersions that have a specific density between 1 and 2 g/ml and a viscosity of between 100 and 500 cps can be processed.
DE 40 40 150 C2 likewise describes a process for the even coating of a honeycomb carrier made out of ceramic or metal. For this process, the honeycomb carrier is submerged in a dipping barrel and filled with the coating dispersion from below. Afterwards, the honeycomb carrier is emptied again through blowing or suction. The honeycomb carrier is then removed from the dipping barrel and cleared with suction or blowing in a separate system in order to avoid blocked flow channels. With this process, coating dispersions with solids content of between 48 and 64% and viscosities from 50 to more than 100 cps can be processed.
Exhaust catalytic converters for internal combustion engines are subject to continually increasing statutory requirements in relation to their conversion of pollutants and service life. An increase of the service life of the catalytic converters can be achieved through an improved catalyst recipe as well as through an increase of the amount of the catalytically active components on the catalyst. However, a higher quantity of catalytically active components also requires a higher loading of the catalyst carrier with coating dispersion. Improved conversion of pollutants can also be achieved through catalyst carriers with higher cell densities. In both cases--with the increase of the coating concentration as well as with the increase of the cell density--the danger in the coating process of clogging of the flow channels with coating dispersion increases.
It is therefore an object of the present invention to make available a new coating process for ceramic and metallic catalyst carriers in honeycomb form that distinguishes itself by the following characteristics:
reproducible loading of the catalyst carrier of a production lot always with the same quantity of coating dispersion PA1 coating of the catalyst carrier with more than 200 g of dry mass per liter of catalyst carrier volume PA1 coating of catalyst carriers with cell densities up to 250 cm.sub.-2 PA1 as uniform a radial and axial thickness of the coating as possible PA1 assured prevention of clogged flow channels PA1 as great an independence of the process from the rheological characteristics of the coating dispersion as possible PA1 a) Filling of the flow channels with a fill quantity that is about 10% greater than the empty volume of the flow channels, so that the coating dispersion goes over the upper face of the catalyst carrier after completion of the filling cycle, PA1 b) Removal of the excess coating dispersion at the top before emptying of the flow channels and PA1 c) Emptying and clearance extraction of the flow channels through an extraction impulse, which is generated by connection of a vacuum tank with the bottom face of the catalyst carrier,